Housing for a radioactive source

ABSTRACT

The radioactive structure comprises a radioactive source surrounded by a housing. The housing contains a first and second shielding body and a connecting device. The first shielding body has a protrusion which contains a first recess for receiving the radioactive source. The second shielding body has a second recess in one face end which accommodates the protrusion and a conical aperture communicating with the second recess in another face end. The connecting device connects the first shielding body to the second shielding body. When the radioactive source is inserted into the first recess and when the protrusion is located in the second recess, the radioactive source emits radiation primarily through the conical aperture into the environment. The source preferably contains americium which emits gamma radiation. The structure may be used as a motion correction sensor or as a marker in nuclear diagnostic imaging.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to the field of nuclear medicine. Inparticular, it relates to the field of radiation diagnosis and toexaminations of a patient by means of a scintillation camera. Still moreparticularly, this invention relates to a housing or container forcarrying a radioactive isotope which housing is to be attached to thepatient's body, the isotope thereby serving as a motion detector or asan anatomical marker for nuclear images.

2. Description of the Prior Art

In nuclear medicine, a radioactive tracer such as technicium isadministered to a patient undergoing examination, and the distributionof the tracer in the patient's body is viewed by aid of a scintillationcamera.

A problem associated with this kind of nuclear examination is themovement of the patient. When the patient moves during examination, ablurred image may result.

In order to overcome this disadvantage, a diagnostic motion correctionscheme has been developed, see U.S. patent application Ser. No. 324,090,filed by Mark W. Groch and James T. Rhodes, under the title "MotionCorrection Circuitry and Method for a Radiation Imaging Device", on Dec.1, 1981. The disclosure of this application is incorporated herein byreference.

According to this motion correction scheme, a blurring of e.g. ^(99m) Tcgated blood pool images due to patient motion is corrected. This isaccomplished by introducing a special radioactive point source, inparticular a gamma ray emitting radioisotope, whose energy window liesoutside that of ^(99m) Tc, into the field-of-view of the scintillationcamera. The point source remains outside of the patient's body; thus, nogamma rays emitted from the source are scatter events. Then the centroidof the point source is monitored. When a change in the centroid of thepoint source is detected due to patient motion, the ^(99m) Tc events arecorrected and repositioned to take into account the motion artifact. Incardiac studies, for instance, the movement of a special radioactivesource which is fixed to the chest of the patient is detected. Thismovement is subtracted from the detected radiation coming from thetracer isotope of different energy signature flowing in the bloodthrough the heart. Thus the "dual isotope motion correction scheme"eliminates the motion blur in the images as they are acquired.

There are two problems that one must keep in mind when using such acentroid (point) source external to the patient's body during imageacquisition. First, the special radioactive point source must be withinthe field-of-view and the events emitted must be detected through thecollimator of the camera. Second, since the point source should beencased in a shielding medium which will attenuate ^(99m) Tc eventsemanating from the patient's body, the point source must not obstructany important anatomical structures in the field-of-view.

SUMMARY OF THE INVENTION

1. Objects

An object of this invention is to provide a housing for a radioactivepoint source which is to be attached to a patient in nuclear medicalexaminations.

Another object of this invention is to provide such a housing whichreliably retains and shields the point source, but leaves an openingfree for emission of radiation towards a radiation detection system suchas a scintillation camera.

Still another object of this invention is to provide a housing for aradioactive point source which can be used in routine nuclearexaminations either as a motion detector or as an anatomical marker fornuclear images.

2. Summary

According to this invention, a housing for a radioactive sourcecomprises a first shielding body, a second shielding body, andconnecting means for connecting the two bodies. The first shielding bodyhas a protrusion which contains a first recess for inserting theradioactive source. The second shielding body has a second recess in itssurface. The second recess is shaped so that the protrusion fitstherein. When the radioactive source is inserted into the first recessand the protrusion is located in the second recess, the radioactivesource will emit radiation into the environment primarily through theconical aperture.

According to a preferred embodiment, a ²⁴¹ Am centroid source is encasedin a disk shaped tungsten alloy holder. The bottom part of the disk isto be laid on the patient so that the top part which contains the gammaray escape aperture, faces away from the patient's body. The escapeaperture is cone-shaped at an angle of e.g. approximately 120° with anexit hole of approximately 2 mm.

The first of the two aforementioned problems is solved by insuring thatthe source is in the field-of-view and that the disk holder not betilted by more than 60° from a plane perpendicular to the collimatorholes.

The second of the aforementioned two problems has a different solutiondepending on the view and collimator used in imaging.

1. RAO with slant hole collimator: The point source should be placedanatomically below the heart and just to the left side of the chest wallcenter.

2. ANT with parallel hole collimator: The point source should be placedanatomically anywhere below the heart, as long as the source facesanteriorly.

3. LAO with parallel hole collimator: The point source should be placedanatomically below the heart and on the central left to far left side ofthe chest wall.

Adhesion of the centroid point source to the patient is preferablyaccomplished by placement of surgical tape in an X-pattern over the topside of the source, adhering the source firmly to the patient, thusinsuring that patient and source motion is unified.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an exploded perspective view of a centroid source holderaccording to the invention, in an enlarged scale;

FIG. 2 is a plane view of the assembled source holder illustrated inFIG. 1;

FIG. 3 is a side view of the assembled source holder illustrated in FIG.1;

FIG. 4 is a plane view of the top of the source holder illustrated inFIG. 1;

FIG. 5 is a side view of the top of the source holder illustrated inFIG. 1;

FIG. 6 is a plane view of the bottom of the source holder illustrated inFIG. 1; and

FIG. 7 is a side view of the bottom of the source holder illustrated inFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1-7, a holder, container or housing containing or carrying aradioactive source 2 is illustrated. The source 2 comprises aradioactive isotope which is used in a diagnostic motion correctionscheme. Thus, the housing may be termed a dual isotope motion correctioncentroid point source holder. In the present case, the source 2 iscommercially available for nuclear medicine purposes. In particular, itis a gamma radiation source available from The Radiochemical Centre,Amersham, England. A 2 mm diameter spherical point source 4 is locatedat one end of a closed-end cylindrical stainless steel housing 6 of 3 mmdiameter and 10 mm length. The other end of the source housing 6 isplugged with an approx. 8 mm long stainless steel plug. The point source4 is a pellet or bead containing the radioactive isotope. For motioncorrection applications, preferably an americium source is used.

The radioactive material americium 241 has a comparatively low energywith regard to the emitted gamma radiation. This energy differs markedlyfrom the energy of gamma radiation emitted by a tracer which isconventionally administered to a patient in nuclear medicine. Thehalf-life of americium is several hundred years. This has the advantagethat during patient examination and during the lifetime of ascintillation camera, a replacement of the radiation source 2 is notnecessary.

An americium point source can also be used as a marker in nuclearimaging by means of a scintillation camera such as the Anger camera.

The housing contains a lower or bottom part 8 and an upper or top part10. Both parts or sections 8 and 10 are made of a shielding material,preferably of a metal. A compound containing tungsten is preferred. Atungsten alloy known as "Mallory 1000" has been found especially useful.The holder sections 8 and 10 are of a material containing tungstenrather than lead because of its durability and its lighter weight.However, also other radiation attenuating materials may be used.

The bottom part 8 represents a first radiation shielding body. As can beseen in FIG. 1, it contains at its upper end a protrusion 12. Theprotrusion 12 and the base portion 14 of the bottom part 8 are both ofcylindrical or disk shape. The protrusion 12 contains a first recess orchamber 16 for holding the cylindrical source housing 6 containing theradioactive americium source 4 therein. In particular, the first recess16 is a channel extending from the periphery of the cylindricalprotrusion 12 to its middle section. Here it merges into another channelor chamber 18 which is arranged perpendicularly thereto. The channels 16and 18 are shaped so that the point source 4 becomes positioned in thecenter of the disk structure 8. Both channels 16, 18 extend from theupper face end of the protrusion 12 to the upper end of the base portion14. In operation, the source 2 is kept in the channel 16 by means of acement.

The top part 10 represents a second radiation shielding body. On itslower end it contains a second recess 20 which is illustrated in FIG. 1in broken lines. The recess 20 as well as the top part 10 have acylindrical or disk shape. The dimension of the second recess 20 is suchthat the protrusion 12 of the bottom part 8 fits tightly therein. In theupper surface of the upper part 10 is provided a conical recess 22. Theconical angle of this recess 22 is designated by α. The recess 22 mergesinto a central aperture 23 connecting the upper recess 22 with the lowerrecess 20. The diameter d of the central aperture 23 is preferably d=2mm, see FIGS. 4 and 5. The aperture 23 and the recess 22 are providedfor permitting the passage of radiation quanta from the point source 4at angles between 0° and 60° towards the upper surface.

To the lower end of the lower part 8, there may be attached a label 25,see FIG. 3. This label 25 contains information such as radiationidentification and a caution note.

There are also provided means for connecting the top part 10 to thebottom part 8. These means comprise two openings 24 extending axiallythrough the bottom part 8 and two openings 26 extending axially throughthe top part 10. These openings 24, 26 are provided at the rim portionsof the parts 8 and 10, respectively, and they are aligned with respectto each other. The connecting means may further comprise rivets 28,extending through the openings 24 and 26 as illustrated in FIG. 3. Thus,in the preferred embodiment the parts 8 and 10 are kept together by therivets 28 running through boxes at diagonally opposite sides of theparts 8 and 10.

During assembly of the structure, the radiation source 2 is inserted andcemented into the first recess 16 and subsequently the protrusion 12 isplugged into the second recess 20. Now the lower end face of the upperpart 10 joins the upper end face of the bottom portion 14. Then the toppart 10 is connected to the bottom part 8 by means of the aforementionedrivets 28. In the closed status of the parts 8 and 10, the radioactivesource 2 will emit gamma radiation basically through the conicalaperture 22. The centroid source holder may now be attached to thepatient. The openings 22, 23 in the center of the upper part 10 serve asa port to pass the emitted radiation from the point source 4 towards thescintillation camera.

After assembly, the upper and lower parts 8, 10 both enclose the source2 in a tight manner. The point source 4 is completely encapsulated withthe exception of the area where the aperture 23 and the conical recess22 are provided. Due to this feature, an optimum shielding of thepatient and of the technician or physician performing the examination isachieved.

Examinations have proven that a wide range of conical angles α can beused. However, an angle of approximately α=120° has proven to be bestfor certain applications. This angle α is especially useful if a movablescintillation camera is used. It ensures that the scintillation cameraexerts a constant count rate at various positions of the camera withrespect to the housing 8, 10.

In order that the angle α can be used optimally, the height b of theprotrusion 12 should not markedly exceed the diameter of the source 2.

Thus, the 120° angle is empirically chosen. It allows for angularplacement of the camera head relative to the source. Yet it is not toowide an angle to prevent shielding of the radiation from the patient andsurrounding environment (nurses, etc.). A vertical walled opening wouldnot permit many counts to reach the camera head if the head is placed atan angle because the parallel collimator would not accept rays at anglesto the axes of the collimator paths.

The size of the container is small so that it can be easily taped, e.g.to the chest of a patient by a surgical tape. It will move with themovement of the patient's chest. In a preferred embodiment, thecontainer dimensions are A=2.5 cm; B=1.9 cm; a=7 mm; b=3 mm; c=4 mm.

While the forms of the housing for a radioactive source herein describedconstitute preferred embodiments of the invention, it is to beunderstood that the invention is not limited to these precise forms ofassembly, and that a variety of changes may be made therein withoutdeparting from the scope of the invention.

What is claimed is:
 1. A housing for a radioactive source which is to beattached to a patient, comprising in combination:(a) a first shieldingbody having a protrusion, said protrusion containing a first recess forinserting said radioactive source therein, (b) a second shielding bodyhaving(b1) a second recess in the surface thereof, said second recesshaving a shape such that said protrusion fits therein, and (b2) aconical aperture communicating with said second recess, said radioactivesource emitting radiation through said conical aperture when said sourceis inserted into said first recess and when said protrusion is locatedin said second recess; and (c) connecting means for connecting saidfirst shielding body to said second shielding body when said radioactivesource is inserted into said first recess and when said protrusion islocated in said second recess, said radioactive source thereby emittingradiation into the environment primarily through said conical aperture.2. The housing according to claim 1, wherein said first and secondshielding bodies are of cylindrical shape.
 3. The housing according toclaim 1, wherein said protrusion and said second recess are ofcylindrical shape.
 4. The housing according to claim 1, wherein saidconical aperture has a cone angle of approximately 120°.
 5. The housingaccording to claim 1, wherein said first and second shielding bodiescomprise a metal.
 6. The housing according to claim 5, wherein saidmetal contains tungsten.
 7. The housing according to claim 1, whereinsaid radioactive source contains americium.
 8. The housing according toclaim 1 for a radioactive source having the form of an elongated rod,one end of said rod containing a radioactive substance, comprising achannel-shaped first recess in said first shielding body for mountingsaid radioactive source therein.
 9. The housing according to claim 8,wherein said protrusion and said second recess are of cylindrical shape,wherein said channel-shaped first recess is open at one end, and whereinsaid rod end containing said radioactive stubstance is located in thecentral portion of said cylindrical protrusion.
 10. The housingaccording to claim 9, wherein said channel-like first recess merges intoa channel-like third recess located in the central portion of saidcylindrical protrusion.
 11. The housing according to claim 1, whereinsaid second shielding body has a first end face and a second end faceopposite to said first end face, wherein said second recess is providedin said first end face, and wherein said conical aperture is provided insaid second end face.